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Why nitrogen content must be half that of aluminium in steel


Frequently Asked Questions

Nitrogen plays a significant role in promoting strain age embrittlement in base steels and welds. Strain ageing arises from interactions between free nitrogen (and/or carbon) atoms in the steel and dislocations in the atomic lattice. Dislocations are two dimensional (line) defects, around which the lattice order is disturbed. Dislocations are responsible for nearly all aspects of the plastic deformation of metals. Nitrogen (and/or carbon) atoms diffuse to dislocations and anchor the dislocations in a certain position so that an 'activation energy' must be exceeded before plastic strain can occur. The return of a yield point, increase in yield stress after ageing, decrease in ductility, a shift in impact transition temperature and a low value of strain-rate sensitivity are all symptoms of strain ageing.

The addition of aluminium to the steel can tie up the free nitrogen through the formation of aluminium nitride (AlN). However, the aluminium must be available to combine with nitrogen. Aluminium has a higher affinity for oxygen than nitrogen, and so in the presence of oxygen and nitrogen will form Al2O3 in preference to AlN. The oxygen levels in a weld are more difficult to control than in the base steels, and so less AlN may form, and some free nitrogen may remain.

Nitrogen can also cause porosity in welds, appearing when the nitrogen content exceeds 0.045% mass. Again, the addition of aluminium can inhibit this problem.

The requirement for aluminium content to be greater than twice the nitrogen content is to ensure that there is some free aluminium available to combine with the free nitrogen. Certain chemical analysis methods can distinguish between free and precipitated aluminium, and the value of free aluminium is usually used to meet this criterion. However, some work has shown that the aluminium precipitated as AlN is approximately constant, irrespective of the aluminium content above a threshold value. (See Knowledge Summary The effect of aluminium on the microstructure and toughness of submerged arc welds in C-Mn-Nb steel.).

If the free aluminium value does not exceed twice the nitrogen content specified, the value of total aluminium should be used. It is considered that aluminium content above approximately 0.075% will tie up all the available nitrogen, and that an increase in aluminium above this value will give lower than optimum toughness in the base steel.

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